Oh boy oh boy, Spend all day at school, and now I get to put on my writer hat!
@Gwen: I actually really enjoyed your story, and though it was very well written. In fact, nothing about it really jumped out at me as felling wrong(And I really, really don't mean to be offensive to anyone else who's work I've critiqued before), which is sort of rare. Well, I shouldn't say that, there was one thing: I found a couple of issue with the characters voices. Even then, it's never consistent or even big, It's just little choices in language that I don't think they would use - Rarity calling Twilight by her full name, for example, I don't think she's done that in the show since she first met Twilight. I also think you might be playing AJ's accent a bit off; It's close, very very close, but the slang just doesn't seem right to me. But that's definitely one of the hardest things to do in writing, so I wouldn't worry
too much about it.
As for the plot itself, you've got me really interested. You created a really good, semi-spooky atmosphere with the fog, and I love how you handle the pacing of it - you make us wonder what it is, and you let us know that it will be a big issue, or part of it later, but you don't simply launch us into the conflict like someone else might. You've written it like a warning, we have time to take in the characters, but we don't get bored because we know something is going to happen soon. One thing I do have to wonder, and only because it's so early, is how Celestia and Luna are going to factor into it. Because I'm concerned that they might not come back, which would make their appearance in the beginning feel extremely off.
@Rich: Not...really sure where to start on this one.
Now I don't say that because it was bad, mind. But it's a very,
very different interpretation of Luna than I'm used to. Because, unless I'm taking it differently from how you intended, it seem like Luna was pretty bitter right off the hop. And while it's a valid idea, and 1000 years is a long, long time, it just doesn't strike me as meshing with the repentant Luna we saw at the end of the second episode. Of course, this is all told from a later perspective, so it may just be the way she is 'now' colouring the way she was 'then' - and unreliable narrator sort of ordeal, which is pretty good. At either rate, you've done an excellent job with creating a strong feeling, I felt a lot of tension come the end of it.
There's not much I can say about the prose of it, really. It's not bad, but it's not really spectacular, it's just sort of there. Now that's not a problem, it's really only there to move the story along, but I've always loved my prose to have a sort of feel in and of itself - whether its Pratchett-esque in it's offhanded snark, or rhythmic and flowing. It feels like your going for a sort of rhythm as well, with the occasional single line paragraphs, and I just don't feel like you made it all the way. Even if you weren't going for that, I do recommend thinking about it - it really lends to a story in general, not to mention that Nightmare Moon's dialogue had a real sort of elegance to it in the show, and since it's being told from her perspective (or so I assume, judging from the bitter voice), I think it would help the voice a lot. Of course in the end, that's just my personal preference.